How to define the return type / OutputType of a function

Why is the following changing type?

function SomeFunction($SomeParameter){    return $SomeParameter}

I guess I need to set a return type, but how?

An example is using:

$NewFolder=Join-Path $CurrentFolder -ChildPath $FolderName$Tmp=SomeFunction($NewFolder)

Now $Tmp is an array and not just a path